Digester control systems



Oct. 20, 1959 A. E. BENNETT v DIGESTER CONTROL SYSTEMS 3 Sheets-Sheet 1Filed Jan. 5, 1956 (1ttomeg Oct. 20, 1959' A. E. BENNETT 9,

I DIGESTER CONTROL SYSTEMS Filed Jan. 3, 1956 5 Sheets-Sheet 2 Amok/5wE. BEAM Err VB L14. M

(Ittorneg Oct. 20,1955

A. E. BENNETT 2,909,239

DIGESTER CONTROL SYSTEMS Filed Jan. 3. 1956 3 Sheets-Sheet 3 InventorANDREW E; BENNETT BB c wM U it d tate m Q 2,909,239 DIGESTER CONTROLSYSTEMS Andrew E. Bennett, We'ym'outh, Mass., ass'ignor, by mesneassignments, to Worthington Corporation, New York, N.Y., a corporationof Delaware Application January 3, 1956, Serial No. 556,958 4 Claims.(Cl. 18361) This invention relates to the control of wood pulp digestersand particularly concerns means, for use in connection with the reliefof gases, for clearing a relief line screen in the digester, said screenbeing commonly employed to prevent solid and semi-solid material fromentering the relief conduit.

In the manufacture of wood pulp, it is customary to fill a digester withwood chips, of a character suited to the product, to which is added acharge of cooking liquid of a selected volume, and thereafter to set upa reaction by the admission of steam or other heating agent. It iscommon practice to raise the cook temperature, as rapidly as possiblewithout impairing the product, to a selected maximum value in accordancewith a predetermined vapor-pressure-temperature curve, at which valuethe cook temperature is preferably maintained until the pulp hasattained the' required fiber separation.

In the course of this cooking process, gases in the form of air andvolatile material, such as turpentine or the like, are given off, and itis usual to vent these from the top of the digester as required tomaintain predetermined temperature and pressure values therein. It willbe understood that the eflicient operation of a digester depends uponpenetration of the cooking liquor into all of the chips, and that thisin turn depends upon proper circulationthroughout the entire mass. Thecharacteristics of the cook mass are such that circulation tends to beextremely sluggish, particularly during the early stages of the cook,and due to the chemical constituents of the liquor solution, and to thereaction taking place by the application of heat, there is a tendencytoward foaming which under certain conditions results in carrying liquorover into the relief line and lodging masses of solid or semi-solidmaterial against the screen, thereby clogging the screen and seriouslyinterfering with the operation of the cooking process. It thereforebecomes necessary to clear the screen as soon as it becomes clogged, andfor this purpose it has been customary to blow steam back through therelief line into the digester to free the screen of clogging matter. 7

I am aware that various means have been utilized for clearing thescreen, such as by employing a hand operated steam valve, or automaticdevices for manipulating the valve. Among such automatic devices it hasbeen proposed to measure the pressure differential across the screen,and when the differential pressure has reached a predetermined value, apurging cycle is introduced by opening a steam valve and closing arelief valve under the control of instrumentalities which includeelectrical connections and a time-relay, whereby steam or other purgingfluid is admitted to the relief line to purge the screen for a selectedperiod of time. If the screen is not cleared, the cycle is repeated.

The difiiculty with screen purge or blow-back systems as utilizedheretofore, so far as known to me, is that the blow-back medium entersthe dome of the digester in excessive amounts, thereby raising the dometemperature and pressure which results in disrupted circulation.Furthermore, when the purge cycle is terminated and digester relief isagain established, the excessive steam must be relieved which increasesthe load on the vapor condensing system. This is due to the fact thatthere can be no predetermined time for a purge cycle to function as itPatented Oct. 20, 1959 2 should, since a proper purge cycle varies withthe existing digester pressure and has no relation to a definite uniformtime period.

I have discovered that a surge of pressure on the relief line side ofthe digester screen resulting in a hammerlike shock, the force of whichexceeds the digester pres sure, not only is very effective to clear thescreen, but does not interfere with the digester pressure, temperature,or circulation, provided the force of the shock is controlled andimmediately terminated. As is hereinafter more fully pointed out, thisis accomplished by means, embodying this invention, which respondimmediately under some cook conditions to a selected pressure dropacross the screen, and under other cook conditions to varying selectedpressure drops across the screen to close: the relief valve and open thepurge valve to introducea blast of steam into the relief line, thusraising the relief? line pressure sufliciently to provide the necessaryshock and thereafter functions to abruptly close the purge valve. andopen the relief valve to complete the purge'cycle be-- fore the digesterpressure, temperature, and circulation; have been affected. If thescreen has not been sufficiently cleared by the blow-back cycle, thepurge is immediately" repeated until the screen has been adequatelycleaned.

Experience has indicated that a series of repetitive short:

duration pressure pulses is more effective in clearing the; screen'thana prolonged purge.

In actual practice, it has been found that since circula tion of thecook mass is particularly sluggish during: the initial stages of thecook, there is a greater tendency at this part of the cook cycle forsurging and foaming to occur which tend to carry over into the reliefline solid and semi-solid matter, thereby causing the screen to becomeclogged. On the other hand, I have found that as the digester pressureapproaches and arrives at the maximum pressure utilized during theholding period of the cook, while the circulation is less sluggish thanat the early stages, there is a tendency for the pressure to fluctuatesomewhat due to various causes depending on the type of digesteremployed and the constituents which make up the cook mass. Thus thecooking conditions change during the cooking process and when required,in order to meet this situation and provide effective means for clearingthe screen under these changing conditions, I govern the operation ofthe purge valve and relief valve by means which respond to a pressuredrop across the a screen. The means include a differential pressurerespouse mechanism combined with a ratio adjustment for said mechanismand with mechanism which provides a so-called differential gap-action,whereby the pressure drop across the screen required to initiate thepurge cycle is gradually increased during the pressure rise period, andwhereby the purge cycle is terminated at a selected pressure in excessof the digester pressure at the initial stage of the cook, which excesspressure is gradual- 1y reduced to some selected value at the maximumpressure maintained in the digester during the holding period. Thus at adigester pressure of 5 p.s.i., for example, and a gap-action range of 20p.s.i., the purge cycle may be initiated when the relief line pressuredrops to 0 p.s.i. and the cycle may be terminated When the relief linepressure reaches 20 p.s.i. By this means the cycle is initiated at theavailable relief line pressure drop and is terminated when the reliefpressure is suiiiciently in excess of the digester pressure to clear thescreen at a time in the cook period when there is the greatest tendencyfor the screen to become clogged. On the other hand, when thedigesterpressure reaches its maximum value, for example IOO p.s.i., thecontrol mechanism may be arranged so that the purge cycle is notinitiated until the relief pressure 7 drops to p.s.i. and is terminatedwhen the relief pres-- sure reaches p.s.i. Thus the pressure droprequired to initiate the purge cycle is such that fluctuations which mayoccur in the digester pressure at this period of the cook do not causeso-called false blow-backs. Furthermore at this stage of the cook,better circulation is obtained so that there is less tendency for thescreen to become clogged, and again the consistency of the pulp is suchthat less excess relief pressure is required to clear the screen.

While it is preferable to control the purge cycle in the manner justdescribed, the control mechanism embodying my invention may be arranged,when conditions permit, to initiate the purge cycle at a selectedpressure drop across the screen and to terminate the cycle at a selectedexcess of relief line pressure above the digester pressure, which dropin pressure and excess of pressure remain constant throughout thedigester pressure rise period.

It is an object of this invention to provide means for clearing thescreen of a pulp digester, whereby when the screen becomes sufiicientlyclogged to interfere with the relief of volatile fluids, the reliefvalve is closed and purging fluid under pressure is admitted to therelief line to provide to the screen a hammer-like shock withoutintroducing purging fluid into the digester in quantities sufficient tointerfere with the cooking process.

It is an object of this invention to provide means for clearing thescreen of a pulp digester, whereby when the screen becomes suflicientlyclogged to interfere with the relief of volatile fluids, the reliefvalve is closed and purging fluid under pressure is admittted to therelief line to provide a sudden increase in relief line pressure, whichincrease exceeds the pressure obtaining in the digester by a selectedamount, the admission of purging fluid being then abruptly terminated,whereby the screen is subjected to a hammer-like shock for the purposeof clearing the screen and the abrupt termination of purging fluidprevents sufficient purge fluid from entering the digester to interferewith the cooking process.

It is an object of this invention to clear the screen of a pulp digesterby means which respond to varying pressure drops across the screenduring the pressure rise period of the cook to close the relief valveand admit purge fluid under pressure to the relief line, therebyproviding relief line pressures which exceed the digester pressure byamounts which vary during the said pressure rise period.

It is still another object of this invention to provide purge cyclecontrol mechanism which functions to govern a purge valve and reliefvalve to initiate a purge cycle for the purpose of clearing the screenand, if necessary, to repeat said cycle until the screen has beencleared, at which time the relief valve may again be manipulated by anysuitable means for governing the relief of volatile fluid throughout thecooking process.

These and other objects of this invention will be more fully understoodfrom the following description when taken in connection with theaccompanying drawings, wherein:

Fig. 1 is a view, partly in diagram, of a pulp digester of the directsteam heated type, showing automatic blowback mechanism, embodying myinvention, for clearing the digester screen, together with means forgoverning the steam input to the digester and with means for varying therelief of volatile fluids therefrom;

Fig. 2 is an enlarged fragmentary view showing the gas relief screenmounted in the neck of the digester;

'Fig. 3 is a front elevation, enlarged and partly in diagram, of thecontroller shown in Fig. 1 for governing the blow-back mechanism;

Figs. 4 and 5 are details of a portion of the mechanism shown in Fig. 3,illustrating various operating positions; and

Figs. 6 and 7 are graphs showing the effect of'eertain adjustments whichmay be made with the control mechanism shown in Fig. 3.

Having reference to the drawings and particularly to Fig. 1, there isshown a digester, generally indicated at 10, which includes a bodyportion 11, a cone-shaped bottom '12, a dome 1 3, and a neck 14, mountedon the dome, having the usual detachable cover 15 to provide forcharging the digester with wood chips. The bottom 12 has connectedtherewith a pipe 16 equipped with a hand valve 17 through which heatedcooking liquor may be introduced and the digester filled to a level asindicated, for example, by dotted line 18. The pipe 16 and valve 17 arealso utilized to discharge the finished pulp at the end of the cook.

The digester 10 is provided with a relief line 20 which is connectedwith the neck 14, the neck being herein shown as including a removablescreen 21. As more clearly shown in Fig. 2, the screen 21 may be ofcylindrical shape and at the lower end may have an annular flange 22secured thereto which is adapted to rest on an internal flange 23,suitably attached to the neck 14. The upper end of the screen 21 has anannular flange 24, also secured thereto, which engages the inner surfaceof a gasket 25 disposed between the cover 15 and the neck 14 to whichthe cover may be secured, as by stud bolts 26.

The relief line 20 includes a relief valve 27 which may be actuated by adiaphragm 28 partially defining a pressure chamber 29, said chamberbeing in connection with a three-way pressure operated valve 30, bymeans of a pipe 31. The relief valve 27 is herein shown as of thereverse acting type, and preferably includes a double seated valvemember 32 which is connected with the diaphragm 28 by means of a stem33, pressure in the chamber 29 being opposed by the usual compressionspring 34. The three-way valve 30 has a valve member 35 which cooperateswith oppositely disposed valve seats 36 and 37 and may be actuated by adiaphragm 38, partially defining a pressure chamber 39, to which theoutput pressure from a controller 40 is connected by means of a pipe 41.The diaphragm 38 of the three-way valve is connected with the valvemember 35 by a stem 42 and is backed by the usual compression spring 43.The threeway valve 30 has three connections, namely, a connection 44 foroperative connection with the relief valve 27, a connection 45 forreceiving a regulated supply of operating fluid pressure, and aconnection 46 to atmosphere. The connection 45 for receiving operatingfluid pressure communicates through a pipe 47 with a pressure reductionvalve 48 in turn connected with a source of operating fluid pressure,not shown, by means of a pipe 49. The valve 48 may be provided with theusual valve member 50 which cooperates with a seat 51 and functions tovary pressure through the valve as governed by a diaphragm 52. Thediaphragm 52 responds to changes in pressure in an outlet chamber 53 ofthe valve and functions to maintain a selected pressure in the pipe 47as determined by a compression spring 54, disposed at the upper side ofthe diaphragm, the tension of which may be suitably varied by ahandwheel 55 to provide varying pressures, as indicated by a suitablepressure gauge 56, for actuating the relief valve 27. The three-wayvalve 30 is so arranged that the pipes 47 and 31 are normally connected,and the connection 46 with atmosphere is closed. Thus unless outputpressure from the controller 40 is applied to the three-way valvediaphragm 38, the relief valve 27 may be manually governed by thereduction valve 48.

While I have herein shown a manually operated pressure reduction valve48 for governing the relief valve to vary the relief of gases from thedigester as required for the effective operation of the cooking process,it will be understood that instead of the manually operated pressurecontrol valve, any suitable form of automatic gas relief controlmechanism may be utilized for the purpose.

The relief line 20 has connected therewith a blow-back line 59 whichcommunicates with a supply under pressure, not shown, of a suitablepurge fluid, such as steam, for

aeoaasa the purpose of clearing the screen 21 when it becomes cloggedwith solid or semi-solid material. The flow of purge fluid through theblow-back line 59 is governedby a valve 60 which may be generallysimilar in construction to the relief line valve 27. The purge valve 60is also preferably of the reverse acting type and includes a diaphragm61 which partially defines a pressure chamber 62 connected with theoutput pressure line 41 from the controller 40 by means of a pipe 63.Pressure in the chamber 62 is opposed by a compression spring 64 whichsurrounds a stem 65 connecting the diaphragm 61 with a suitable valvemember, not shown. As is the case with the relief valve 27, theblow-back valve 60 is normally closed by the spring 64, is opened on anincrease of pressure in the diaphragm chamber 62, and functions togetherwith the relief valve, as governed by the controller 40, to initiate andterminate purge fluid cycles for the purpose of clearing the screen 21in a manner to be described.

The digester is also provided with a steam admission line 70 whichconnects a source of steam supply under pressure, not shown, with aperforated ring type manifold 71, or an equivalent steam distributiondevice disposed in the upper portion of the digester bottom '12, bymeans of a plurality of pipes 72. Steam admission to the digesterthrough the pipe 70 may be varied by a valve 73 as governed by a timecycle pressure controller 75 which responds to pressure changes,preferably at the bottom 12 of the digester, through a pipe 76, andfunctions to raise the cook pressure to a selected maximum value inaccordance with a predetermined pressure rise curve, and to maintain thepressure at said selected maximum value over a predetermined period oftime. For this purpose, the controller 75 may be of any well-knownconstruction, and as herein illustrated is of the pneumatic type. Itreceives a supply of operating fluid under regulated pressure through apipe 77 from a source, not shown, and responds to changes in digesterbottom pressure to vary output pressure to the diaphragm 78 of the steamvalve 73 to vary the digester pressure in accordance with thepredetermined cook cycle. Pressure from the controller 75 actuates thevalve 73, and for this purpose is connected by means of a pipe 80 with apressure chamber 79, partially defined by the diaphragm 78. Thediaphragm 7-8 is backed by the usual compression spring 81 and, by meansof a stem 82, actuates a valve member, not shown, to vary steam flow tothe digester as required to conform to the cook schedule. And while Ihave herein shown a preferred form of steam admission control mechanismfor governing the digester pressure throughout the cook cycle, it willbe understood that any suitable'means may be utilized for this purpose.

The controller 40, which governs the blow-back cycle, responds tovarying pressure drops across the screen 21 by means of a pipe 83,connected with the dome 13 of the digester upstream of the screen, andby a pipe 84 which connects with the relief line 20 downstream of thesaid screen. The controller combines a differential pressure responsemechanism with differential gap-action mechanism, whereby under somecook conditions, the blow-back valve 60 and relief valve 27 may beactuated to initiate a purge fluid cycle when a selected pressure dropoccurs across the screen 21, and may be again actuated to terminate thecycle when the relief line pressure exceeds the digester pressure by aselected value. However when the cook conditions require that thecontroller functions to initiate the purge cycle when selected pressuredrops occur across the screen 21, which pressure drops increaseproportionally with the increase in digester pressure, and saidconditions also require that the cycle be terminated when the reliefline pressure exceeds the digester pressure by selected amounts WhlChdecrease proportionally with the digester pressure, I provide a ratioadjustment means for the pressure diiferential responsive mechanism,which means function together with the said mechanism and with thedifferential gap-action 8 device to accomplish this purpose. will now bedescribed.

The controller 40 may be similar to that shown in the United Statespatent to Robins, No. 2,585,347, with pressure differential responsemechanism having a ratio adjustment incorporated therein. When soarranged, the controller referred to will accomplish the objects of thisinvention when applied to a pulp digester to govern purge cyclemechanism. In order 'to simplify this description however, thecontroller shown in Fig. 3 is a partially diagrammatic illustration ofmechanism suited to the purpose. The controller may be mounted in acase, indicated by broken lines 85, having the usual back plate 86 onwhich the mechanism is supported. This coinprises dilferential pressureresponse mechanism including the Bourdon coils and 91, a differentialpressure setting device, generally indicated at 92, a ratio adjustmentfor said mechanism, generally indicated at 88 and a differentialgap-action device, generally indicated at 93. The coil 90 is responsiveto digester pressure by means of the pipe 83, and the coil 91 isresponsive to relief line pressure by means of the pipe 84, each coilhaving a fixed end 94 to which the said pipes connect. The coil 90 has ashaft 95, disposed centrally thereof, with which the free end of thecoil is in operative connection, it being understood that the shaft isrotated by the coil in proportional relation to changes in digesterpressure. Similarly the coil 91 is provided with a central shaft 96which is rotated by the coil in proportional relation to changes inrelief line pressure. The coil 90 may have a pressure range from 0p.s.i. to 150 p.s.i., a range which is ample for digester cooks, and theshaft of the said coil may have secured thereto an indicator 97 which isadapted to cooperate with a scale 98 having a similar pressure range.The pressure relief line coil 91 may also have a range of O p.s.i. to150 p.s.i., and an indicator 99 may be secured to the shaft 96 tocooperate with a suitable scale 100.

The controller 40 operates to indicate the pressure setting of thecontroller in a manner to be described.

The controller 40 functions to maintain the relief line pressure on aselected basis in reference to the digester pressure and in the eventthat the relief line pressure drops below the digester pressure by aselected amount, the coil 91 actuates the gap-action mechanism 93 toinitiate a screen purge cycle over a selected range of relief linepressure change, and thereafter functions to abruptly terminate saidcycle. To this end, the coils 90 and 91 and the index 102 aremechanically interconnected by a differential linkage. The index 102 ispositioned on the scale 101 by means of the digester pressure responsivecoil 90 to indicate the setting of the controller. For this purpose, theindex 102 is mounted on a fixed pivot 103 at its lower end and has apivotal connection 104 with a link 105 midway between the ends of thesaid link. The link 105 has at one end a pivotal connection 106 with theupper end of a link 107, and at the other end is pivoted at 108 with afloating link 109. The link 107 has, between its ends, a pivotalconnection 110 with a sector plate 111 and at its lower end has anadjustable pivotal connection 112 with one end of a link 113 theopposite end of which is pivotally connected at 114 with the lower endof the indicator 97. The lower end of the link 109 is pivoted at 117 toone end of a link 118 the other end of which has a pivotal connection119 with the lower end of the indicator 99. The floating or differentiallink 109 is pivoted at 115, midway between its ends, to one end of alink 116, herein shown broken for clarity, by which the gapactionmechanism is actuated in a manner to be dmcribed.

The gapaction mechanism 93 includes a nozzle 125, a batlle 126 and abellows 127. The nozzle receives operating fluid, such as air underregulated pressure, from a source, not shown, through pipes 128 and 129,

In order to simplify the description, a' third scale 101 is shown withwhich an index 102 cothe former having a restriction 130 of a capacityless than that of the nozzle so that the nozzle pressure is determinedby the relative position of the nozzle and bafile. The bellows 127 has afixed end 131 and a free end 132 to which a post 133 is rigidly secured.The interior of the bellows 127 connects with nozzle pressure in thepipe 129 by means of a pipe 134, and the pipe 129 in turn connects withthe pipe 41 by which the nozzle pressure is communicated to thediaphragm chamber 62 of the steam valve 60 and to the diaphragm chamber39 of the three-way valve 30.

Means for actuating the baflle 126 in respect to a nozzle 125 include anarc-shaped member 135 having at its lower end a pivotal connection 136with the bellows post 133 and at its upper end a pivotal connection 137with the link 116. The bafile 126 is pivotally mounted at 138, betweenits ends, on a fixed post 139, one end of the battle being adapted tocooperate with the nozzle 125 and being normally held in engagement withsame by a compression spring 14% disposed at the opposite side of thepivot 138 and confined between the baffle and a fixed member 141. Meansfor positioning the baffle 126 in respect to the nozzle 125 is in theform of a baffle actuating member 142 rigidly secured to a rotatable arm143 pivotally connected at its upper end 144 to a fixed bracket 145 andat its lower end having a pivotal connection 146 with one end of a link147. The other end of the link 147 is connected by an adjustable pivot148 with the arc-shaped member 135. The radial center of the arc-shapedmember 135 is at the pivot 146 and the adjustable pivot 148 cooperateswith a slot 149 in the arc-shaped member and is provided with lockingmeans 150 for clamping the pivot at any selected position lengthwise ofthe slot.

The gap-action operation can best be visualized from Figs. 4 and 5. Asis well known, gap-action control is obtained by the use of positivefeedback. Let it be assumed, as shown in Fig. 4, that the pivotalconnection 115 of the link 116 has been moved in the direction of thearrow from some position B to position A, at which position the bafileactuating arm 142 permits the spring 140 to cause the bafile 126 tostart to throttle the nozzle 125. The increase in nozzle pressure willstart to expand the bellows 127 which will lift the arm 142 away fromthe baffle, thereby increasing the nozzle pressure to the supplypressure. The parts will then assume the dotted line position shown. Nofurther action takes place until the pivotal connection 115 moves in thedirection of the arrow from position A to some position B, at whichposition the baffle actuating arm 142 will again engage the bafile andstart to decrease the nozzle pressure. This position of the parts isshown in full lines in Fig. 5. The decrease in nozzle pressure starts tocollapse the bellows 127, thereby further uncovering the nozzle andimmediately reducing the nozzle pressure to p.s.i. The parts positionjust referred to is shown in Fig. 5 in dotted lines. It is evident thatthe motion AB of link 116, necessary to change to the nozzle pressurebetween 0 p.s.i. and supply pressure, may be varied by adjusting theposition of the pivot 148 along the slot 149 of the arc-shaped member135, the gap range being increased by moving the pivot 148 towards thepivotal connection 136 of the arcshaped member with the post 133, andbeing decreased when the pivot 148 is moved in the opposite direction.By the proper arrangement of parts, the gap distance AB may be centeredabout point M which is at the center of the gap band, and the point M isalways at the center of the band at any band width to which themechanism may be adjusted.

The differential pressure link 109, shown in Fig. 3, is positionedaround the pivot 117 by the coil 90 in response to changes in digesterpressure, and the link is positioned around the pivot 193 by the coil 91in response to changes in relief line pressure. Since the index 192 ispositioned on the scale 101 by the indicator 97 of the coil on aselected adjustable basis, to be described, each position of the index102 will require an identical position of the relief line pressureindicator 97 to position pivot 115 at the mid point M of thedifferential gap. Thus the index 102 sets the center M of thedifferential gap-action.

As previously referred to, the index 102 is positioned by the digesterpressure indicator 97 on some selected adjustable basis. For thispurpose, as herein shown, the sector 111 is utilized to introduce adifferential setting between the indicator 97 and the index 102. Thesector 111 has a fixed pivotal connection 120 at its lower end and atits upper end is provided with a toothed arc-shaped section 121 whichcooperates with a manually operable pinion 122. Thus when the sector 111is in the mid position shown, the position of the index 102 on the scale101 corresponds with the position of the digester pressure indicator 97on the scale 98. However if the sector 111 is rotated clockwise aboutits pivot, the index 102 is moved up the scale 101 in respect to theindicator 97, and when the sector is turned counterclockwise, the index102 is moved down the scale in respect to the indicator 97. And sincethe index 102 is connected with the gap-action link 116 by means oflinks 105 and 109, the relief pressure coil 91 operates around the indexsetting to provide relief line pressures to initiate and terminate purgecycles when the screen becomes clogged.

As shown in Fig. 3, the sector 111 is at its mid position of the rangeof adjustment, and the pivots 112 and 106 are equidistant from the pivoton which the link 107 is mounted to provide equal leverage arms 151 and152. Therefore the differential pressure setting between the digesterindicator 97 and the index 102 is O p.s.i., and since the leverage arms151 and 152 of the link 107 are equal in length, the position of theindex 102 on the scale 101 coincides with the position of the indicator97 on the scale 98 at all pressures obtaining in the digester throughoutthe cook period. Therefore as shown in Fig. 3, since the differentialpressure setting is 0 p.s.i., the drop in relief line pressure acrossthe screen 21 required to initiate a purge cycle, and the amount ofrelief line pressure in excess of digester pressure required toterminate said cycle, is governed by the width of the differentialgap-action band.

Diiferential pressure settings other than 0 p.s.i. are obtained by themanually operated pinion 122, by which the sector 111 is rotated asdescribed above, which positions pivot 110 thus introducing a constantdifferential between indicator 97 and index 102 over the entire scalerange. The index 102 can be made to lead the indicator 97 or lag behindthe said indicator by moving the pivot 110 to the right, or to the left,respectively. The introduction of 5 p.s.i. leading pressure differentialis illustrated by the graph shown in Fig. 6. The graph shows therelationship of the index 102 to the digester pressure indicator 97throughout the digester pressure rise. The digester pressure is plottedon the abscissa, indicated by the full line X, and the differentialpressure between the indicator 97 and index 102 is plotted on theordinate of the graph. Plus values indicated that the index leads theindicator and minus values indicate that the index lags behind theindicator. The graph illustrates a 5 p.s.i. lead of the index inrelation to the indicator, the position of the index being shown by thedash line Y. If it be assumed'that the controller differential gapsetting is 20 p.s.i., a gap band corresponding to this width isillustrated by the shaded area and shows that the index line Y is at amid position in the band. The upper edge U of the band is at plus 15p.s.i. and the lower edge L is at minus 5 p.s.i. Therefore if the reliefline pressure drops to 5 p.s.i. below the digester pressure at any pointin the scale range, the gap-action device 93 will initiate a purgecycle, and when the relief pressure increases to 15 p.s.i. above thedigester pressure, the gap-action device will terminate the said cycle.With this control adjust ment the purge cycle is initiated at the same 5p.s.i. pressure drop across the screen throughout the digester pres surerise, and also throughout said rise the purge cycle is terminated at thesame excess of relief pressure over the pressure obtaining in thedigester.

In addition to the differential pressure adjustment herein abovedescribed, I provide the ratio adjustment 88. The adjustable pivot 112cooperates with a slot 160 in the lower end of the link 107 and, bymeans of a suitable clamp 161, may be adjusted to any selected positiontherein. Thus when the pivot 112 is adjusted to provide an equal lengthto leverage arms 151 and '152 a one-one ratio is obtained so that themovement of the index 102 is equal to that of the indicator 97. Theratio may be increased or decreased by moving the pivot 112 up or down,respectively, in the slot 160.

The graph illustrated in Fig. 7 shows the controller adjusted to thesame 5 p.s.i. pressure setting as shown in Fig. 6, but with a ratiosetting wherein the motion of the index 102 is 0.9 of the motion of theindicator 97. It will be noted from the graph that the index line Yslopes rom a 5 p.s.i. lead at p.s.i. digester pressure to a p.s.i. lagat 100 psi. digester pressure. The differential gap band as shown by theshaded area is again assumed to have a width of 20 p.s.i. If thecontroller 40 is adjusted as shown in Fig. 7 when the digester pressureis at 5 p.s.i., if the relief line pressure drops to 0 p.s.i., the coil91 will turn the indicator 99 counterclockwise and, by means of thedifferential link 109 will operate the gapaction device 93 to provide asudden increase in output pressure to supply pressure in pipe 41. Thusthe threeway valve 30 will disconnect the diaphragm chamber 29 of therelief valve 27 from the pressure reduction valve 48 and connect thesaid diaphragm chamber with atmosphere, thereby permitting the reliefvalve to close. At the same time the purge valve 60 will be open Wide.The relief line pressure will rapidly rise to 20 p.s.i. to deliver therequired shock to the screen 21 and will thereupon be abruptlyterminated before substantially any steam can enter the digester. As thedigester pressure is increased during the cook cycle, while the gap bandremains at the same Width, the pressure drop across the screen requiredto initiate the cycle gradually increases and the excess of relief linepressure above the digester pressure gradually diminishes. Thus when thedigester pressure reaches 100 p.s.i., the purge cycle is initiated at apressure drop across the screen of 15 p.s.i. or, namely, at a reliefline pressure of 85 p.s.i., whereas the cycle is terminated at an excessrelief pressure of 5 p.s.i., or at 105 p.s.i.

It will be understood from the above that the control apparatusembodying this invention combines means responding to digester pressureand to relief pressure with an adjustable diflferential pressure controlsetting means having ratio adjustment means associated therewith whenrequired by the cooking conditions, and differential gapaction meansgoverned by the aforesaid means for actuating some suitable form ofvalve means for initiating and terminating a purge cycle for the purposeof clearing the digester screen when required without interfering withthe efficient operation of the cook process.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. In a control for a digester having heat input means to vary thepressure in the digester, a relief line for venting gases from thedigester, a screen in the relief line, and conduit means connected tosaid digester downstream of said screen and to a source of purging fluidfor directing said purging fluid against the screen, the combinationwith valve means in said relief line, means operatively connecting asource of operating fluid to said valve means to regulate the venting ofgases from the digester, and valve means in said conduit means normallyclosed, of a control mechanism coacting with the respective valve meansto initiate and terminate a purge cycle for clearing the screencomprising, a first means responsive to fluid pressure in said digester,a second means responsive to fluid pressure in the relief line, linkagemeans operatively inter-connecting said first and second pressureresponsive means to maintain a selected pressure differential betweensaid relief line and said digester pressures respectively, a gap actingcontrol valve means connected to said source of operating fluid and eachof said valve means, and mechanical means connecting said gap actingcontrol valve means to said linkage means so constructed and arrangedthat when the relief pressure drops below the preselected differentialpressure setting the relief valve means will be closed and the purgeconduit valve means opened and the reverse operation of these valveswill occur when the relief line pressure returns to the predeterminedditferential pressure setting.

2. in a control for a digester as claimed in claim 1 including, a ratiomechanism associated with the linkage means for varying the selecteddifferential pressure setting in proportional relation to changes indigester pressure.

' 3. In a control for a digester having heat input means to vary thepressure in the digester, a relief line for venting gases from thedigester, a screen in the relief line, and conduit means connected tosaid digester downstream of said screen and to a source of purging fluidfor directing said purging fluid against the screen, the combinationwith valve means in said relief line, means operatively connecting asource of operating fluid to said valve means to regulate the venting ofgases from the digester, and valve means in said conduit means normallyclosed, of a control mechanism coacting with the respective valve meansto initiate and terminate a purge cycle for clearing the screencomprising, a first means responsive to fluid pressure in said digester,a second means responsive to fluid pressure in the relief line, linkagemeans operatively inter-connecting said first and second pressureresponsive means to maintain a selected pressure differential betweensaid relief line and said digester pressures respectively, a gap actingmechanism including, control valve means having a selected range ofoperating pressure change connected to said source of operating fluid, apositive feedback means responsive to changes in output fluid from saidvalve, means operatively connecting said gap acting control valve tosaid relief valve means and said purge conduit valve means, andmechanical means operatively connecting said linkage means and saidfeedback means with said control valve means to open the purge valvemeans and close the relief valve means at one limit of said operatingpressure range when the pressure in the relief line drops below thepredetermined differential pressure setting and to close the purge valvemeans and open the relief valve means when the pressure in the reliefline exceeds the predetermined differential pressure setting.

4. In a control as claimed in claim 3 wherein said mechanical meansincludes, an arc-shaped member pivotally connected at one end to saidfeedback means and movable therewith, a connecting means between saidarc-shaped member at the end remote from said pivotal end and saidlinkage means, and an actuating arm adjustably connected to the medialportion of said arcshaped member and to said control valve for actuatingsaid control valve in accordance with signals from said linkage means.

References Cited in the file of this patent UNITED STATES PATENTS OTHERREFERENCES Pulp and Paper Manufacture, Stephenson, c, McGraw- Hill,1950, pages 453-483, particularly pp. 456, 464, 466, 468-471, 479-481.

